Intel launched the new 5x5 socketed motherboard standard and now surpasses mini-ITX as the smallest socketed board standard at 5.5 x 5.8 inches / 140 x 147 mm. Though not as small as Intel NUC which feature a BGA processor soldered onto the motherboard, 5x5 shares in some features most notably directly using DC power and having SODIMM slots which are typically found in laptops. While socketed like mini-ITX, 5x5 lacks any PCI or PCI-E slot for expansion cards including video cards.
5x5 also supports 2.5 SATA or M.2 storage. It will accept LGA processors with TDPs of up to 65 watts which currently include several desktop i7 products. Fully featured system design considerations aim for a 0.85 liter sized chassis as low as 39mm in z-height with a minimal heatsink for CPUs with 35W TDP. A larger heatsink and thus enclosure is anticipated for CPUs with 65W TDP and 2.5" drives.
There are currently no 5x5 motherboards available for purchase though designs are expected to appear from the usual board manufacturers in the future.